Production from wells in the oil and gas industry often contains particulates such as sand. These particulates could be part of the formation from which the hydrocarbon is being produced, introduced from hydraulic fracturing, or fluid loss material from drilling mud or fracturing fluids, or from a phase change of produced hydrocarbons caused by changing conditions at the wellbore (Asphalt or wax formation). As the particulates are produced, problems occur due to abrasion and plugging of production equipment. In a typical startup after stimulating a well by fracturing, the stimulated well may produce sand until the well has stabilized, often lasting for several months after production commences. Other wells may produce sand for a much longer period of time.
Erosion of the production equipment is severe enough to cause catastrophic failure. High fluid stream velocities are typical and are even purposefully designed for elutriating particles up the well and to the surface. An erosive failure of this nature can become a serious safety and environmental issue for the well operator. A failure such as a breach of high pressure piping or equipment releases uncontrolled high velocity flow of fluid which is hazardous to service personnel. Releasing such fluid to the environment is damaging to the environment resulting in expensive cleanup and loss of production. Repair costs are also high.
In all cases, retention of particulates contaminates surface equipment and the produced fluids and impairs the normal operation of the oil and gas gathering systems and process facilities. Therefore, desanding devices are required for removing sand from the fluid stream. Due to the nature of the gases handled, including pressure and toxicity, all vessels and pressure piping in desanding devices must be manufactured and approved by appropriate boiler and pressure vessel safety authorities.
In one existing system, a pressurized tank (“P-Tank”) is placed on the wellsite and the well is allowed to produce fluid and particulates. The fluid stream is produced from a wellhead and into a P-Tank until sand production ceases. The large size of the P-Tank usually restricts the maximum operating pressure of the vessel to something in the order of 1,000-2,100 kPa. In the case of a gas well, this requires some pressure control to be placed on the well to protect the P-Tank. Further, for a gas well, a pressure reduction usually is associated with an increase in gas velocity which in turn makes sand-laden wellhead effluent much more abrasive and places the pressure controlling choke at risk of failure. Another problem associated with this type of desanding technique is that it is only a temporary solution. If the well continues to make sand, the solution becomes prohibitively expensive. In most situations with this kind of temporary solution, the gas vapors are not conserved and sold as a commercial product.
Another known system includes employing filters to remove particulates. A common design is to have a number of fiber-mesh filter bags placed inside a pressure vessel. The density of the filter bag fiber-mesh is matched to the anticipated size of the particulates. Filter bags are generally not effective in the removal of particulates in a multiphase condition. Usually multiphase flow in the oil and gas operations is unstable. Large slugs of fluid followed by a gas mist are common. In these cases, the fiber bags become a cause of pressure drop and often fail due to the liquid flow there through. Due to the high chance of failure, filter bags may not be trusted to remove particulates in critical applications or where the flow parameters of a well are unknown. An additional problem with filter bags in most jurisdictions is the cost associated with disposal. The fiber-mesh filter bags are considered to be contaminated with hydrocarbons and must be disposed of in accordance to local environmental regulation.
Hydrocylone or cyclone devices are also known for separating particles from liquid mixture by exploiting the centripetal force. By injecting the liquid mixture into a vessel and spinning therein, heavy or large particles move outward towards the wall of the vessel due to the centripetal force, and spirally move down to the bottom of the vessel. Light components move towards the center of the vessel and may be discharged via an outlet. However, Hydrocylone devices have difficulty in separating particulates from effluents with more than two phases, and have an associated pressure drop issue that is undesirable in many oilfield situations.
In Canadian Patent Number 2,433,741, issued Feb. 3, 2004, and in Canadian Patent Number 2,407,554, issued Jun. 20, 2006, both assigned to the Applicant of the subject patent application, a desander is disclosed having an elongate, horizontal vessel with an inlet at one end and an outlet at the other end. As shown in FIG. 1, the desander 10 comprises a cylindrical pressure vessel 11 having a substantially horizontal axis A, a first fluid inlet end 12 adapted for connection to the fluid stream F. The fluid stream F typically comprises a variety of phases including gas G, some liquid L and entrained particulates P such as sand. The fluid stream F containing sand enters through the inlet end 12 and is received by a freeboard portion 13. The freeboard area is set by a downcomer flow barrier, or a weir, 14. Accordingly, the velocity of the fluid stream F slows to a point below the entrainment or elutriation velocity of at least a portion of the particulates P in the fluid stream. Given sufficient horizontal distance without interference, the particulates P eventually fall from the freeboard portion 13. Particulates P and liquids L accumulate over time in the belly portion 15, and the desanded fluid stream, typically liquid L and gas G, emanates from fluid outlet 16.
The accumulated particulates in the vessel require periodical clean-out at sufficient intervals to ensure that the maximum accumulated depth does not encroach on the fluid outlet 16. However, for larger vessels, manual cleaning becomes difficult and time consuming.
While the desanding device disclosed in above Canadian patents has been a great success over the past 11 years or so, improvements are possible. Canadian Patent Application Number 2,799,278, filed on Dec. 19, 2012, and assigned to the Applicant of the subject application, discloses a desander device having a tilted vessel to remove the need for a downcomer flow barrier. However, this desander requires the vessel to be depressurized to remove sand causing downtime and in some cases a hazard for workers as the effluent can contain toxic substances.
Therefore, there continues to exist the desire of further improving the ease with which the vessel can be cleaned and the desire of further improvement in separation efficiency.